Method and apparatus for sorting materials



Dec. 5, 1961 J. F. HUTTER ETAL 3,011,634

METHOD AND APPARATUS FOR SORTING MATERIALS Filed March 5, 1958 4 Sheets-Sheet 1 ATTORNEY 4 Sheets-Sheet 2 LL! 5 I LLI 2 JNV \ITOR wnqa .-QTOR;\'EY

Dec. 5, 1961 J. F. HUTTER ETAL METHOD AND APPARATUS FOR SORTING MATERIALS Filed March 3, 1958 9 m m IIILIIIS w wk S b zma/zd EEEEEE:

Dec. 5, 1961 J. F. HUTTER ETAL METHOD AND APPARATUS FOR SORTING MATERIALS Filed March 3, 1958 4 Sheets-Sheet 3 PIG- | uwmr w lhw II N ATTORNEY Dec. 5, 1961 J. F. HUTTER ETAL METHOD AND APPARATUS FOR SORTING MATERIALS Filed March 3, 1958 4 Sheets-Sheet 4 0.5 dmw wc. mobqmomk O JTOPS Um Ln lllu \TTORXE United States Patent Oihce 3,011,634 Patented Dec. 5, 1961 3,011,634 METHOD AND APPARATUS FOR SORTING MATERIALS James F. Butter and Leonard Kelly,.Bancroft, Ontario, Canada, assignors to K & H Equipment Limited, Toronto, Ontario, Canada Filed Mar. 3, 1958, Ser. No. 718,874 9 Claims. (Cl. 209-74) This invention relates to a method and apparatus for sorting materials.

In many mining operations the valuable minerals are distributed very unevenly and meagerly within the rock mass which is broken as ore. This is so, for example, in the case of most uranium and gold ores where the ratios of ore mineral to gangue are in the order of 1:200 and l:100,000 respectively. Many of the individual pieces of broken rock are, in fact, either completely barren or carry insuflicient ore mineral to meet the cost of milling.

If it were possible to pick out and reject these low grade pieces, a saving equal to the difierence between the cost of picking and the cost of milling could be achieved. Furthermore, if the underground operation could supply sufiicient ore to replace the tonnage of rejected waste, the mill would continue to operate at capacity, but the grade of ore treated and value per ton would be efiectively increased.

While visual hand sorting has been in the past a commonly used concentration measure, it is time-consuming and generally inefiicient.

An object of the present invention is to provide a method and apparatus for sorting materials in an inexpensive and efiicient manner and with much greater speed than has heretofore been possible.

Another object is to provide a high speed method and apparatus for sorting materials which is readily adaptable to treatment of various types of materials such as ores.

The invention resides broadly in a method of sorting materials which comprises providing a stream of pieces to be sorted arranged in a substantially single row, receiving at a point opposite one portion of said stream value identifying radiations from certain of said pieces, directing an air blast across another portion of said stream to change the trajectory of pieces impinged thereby, and controlling the initiation and interruption of said air blast in response to said received radiations.

The invention will be described with reference to the accompanying drawing, in which FIGURE 1 is a diagrammatic view of an arrangement of apparatus in accordance with the invention,

FIGURES 2 to 5, inclusive, illustrate diagrammatically modifications of a portion of the arrangement shown in FIGURE 1,

FIGURE 6 is a sectional elevation of a valve employed in association with the invention,

FIGURE 7 is a block diagram illustrating one system of electronic control, 7 g

FIGURE 7a is a diagram of the output wave forms in respect of a piece of ore,

FIGURE 7b is a diagram of the output wave forms of a piece of waste rock,

FIGURE 8 is a circuit diagram of one form of electronic control system,

FIGURE 9 is a circuit diagram of one form of optical radiation detection system, and 7 FIGURE is a circuit diagram of one form of ferromagnetic detection system.

The invention will be described with particrila'r'refen ence to the concentration of ore, but it will be under-' stood that it may be applied to the sorting of various types of materials.

Referring to FIGURE 1, 1 is a constant speed conveyor belt arranged to receive a supply of rock pieces 2 from any suitable source. The rock pieces are preferably Washed and roughly sized before delivery onto the belt or subjected to any other desired pretreatment.

In the example illustrated, the rock is shown as fed onto a screen 3 where it is subjected to washing sprays 4- following which it falls into a surge bin 5. The fines and wash water may be deposited in a repulper 6 and thence pumped through pipe 7 by pump 8 to the mill grinding circuit. The rock from the surge bin is deposited on the belt in a single row of pieces.

The rock pieces fall oif the end of the belt 1 and it is desirable that the resulting falling stream of rock pieces continue in approximately their normal trajectory and that all the rocks are guided sufficiently to prevent rolling or pitching while in fall. To accomplish this a curved trajectory plate 12 and a flexible dampener 9 mounted on suitable frame members It and 11 are provided to ensure that the pieces of rock are discharge in a substantially vertically aligned row or stream of falling pieces with minimum rolling or pitching motion as indicated at 13.

The lower end portion of the dampener is located in the path of the row of rock pieces on the belt and in slightly spaced relation to the discharge end of the belt. The dampener is formed of semi-rigid or flexible material such as rubber or other suitable plastic composition for yielding engagement by the rock pieces.

It will be apparent that each piece will fall off the end of the belt and accelerate with a constant rate of change of speed, due to gravity, virtually regardless of size. This acceleration in free fall, while the succeeding rock piece is still in contact with the contsarit speed belt, increases the separation between pieces in the falling stream.

The next stage of treatment may be termed the monitoring stage. This stage comprising the scanning of each piece in the falling stream by means of a monitoring device. The type of monitoring device will depend upon the mineral in the ore which it is desired to recover.

if the ore is of radioactive type, such as uranium ore, a sensitive, lead-shielded scintillator counter 14 is employed, this counter being mounted in laterally opposite relation to the falling stream, as shown in FIGURE 1.

-Arranged below the counter 14 is a photoelectric cell 15 having a light source 16 located on the opposite side of the falling stream. Arranged below the cell 15 is an air blast nozzle 17 connected through a pipe 18 to an air receiver 19 and controlled by a valve 20.

An electric circuit 21 connects the counter 14 to the cell 15 through a suitable electronic device 22 which may include an integrating device, time delays, and memory circuits, as required by the particular application, the

V trigger circuit 53 is amplified and inverted, and the leading arrangement being such that the photoelectric cell circuit is broken whenever the radioactive impulses in the detector exceed a predetermined number during the time of rock passage past the detector.

Any suitable arrangement of the type well known to an expert in the electronic field may be employed. One such type of arrangement is illustrated by way of example in FIGURE 7. As indicated therein, the output of gamma radiation counter 14 in the form of pulses of constant amplitude and duration are fed through electric circuit 21 to electronic device 22, which includes an integrating circuit 52, trigger circuit 53, and delay circuit 54.

The pulses from counter 14 are integrated and produce a voltage related to the pulse rate, which in turn is func tionally related to the radioactive mineral content of the passing rock fragment. This'varying voltage is applied to trigger circuit 53, causing a very rapid change of state whenever the input voltage falls below a predetermined amount. The negative going wave-form at the output of ciated amplifier 15, in response to the interruption of the light beam. This pulse is fed to blast control circuit 24 which functions essentially as an electronic relay. A negative going output wave form from and gate 55 is inverted and passed to thyratron 56 only when the input from photoelectric detector and the gate control signal from S.S.M. 54 are both positive. This condition occurs only when a piece of waste rock is interrupting the light beam. Thyratron 56 will pass current and energize valve 7 in response to this positive-going control signal, releasing a blast of high pressure air through nozzle 17. Because the length of blast is governed by the length of time the light beam is broken, the piece of waste rock will receive. a blast of air lasting the full period it is opposite the nozzle, regardless of size. This force applied to the piece of rock gives a new trajectory as indicated at 25, FIG. 1.

If the piece of rock is ore, i.e., contains suflicient mineral to warrant further treatment, S.S.M. 54 is triggered over during the rock passage past detector 14 and the resulting negative control signal by closing gate 55, prevents the photoelectric circuit from firing thyratron 56 and energizing the air valve. No blast of air is discharged through nozzle 17, and the piece of rock therefore continues in its normal path undisturbed. The on-time of S.S.M. 54 is adjusted so that an ore decision obtained from the leading edge of the longest piece of rock will close gate 55 until the trailing edge of that rock has passed the photoelectric detector.

FIGURE 8 illustrates one obvious example of the circuitry involved in the arrangement set forth. A scintillation crystal or phosphor 57 is optically coupled to photomultiplier 58. The output from photomultiplier 58 is in the form of negative going pulses produced by scintillations whose rate of occurrence is dependent on the radiation field in the'crystal. These negative pulses are fed through amplifier inverters 59 and 60 to trigger a conventional single stable state multivibrator (S.S.M.) consisting of triodes 61 and 62, the at res condition of which iswith triode 61 fully conducting. The trigger pulses produce rectangular negative constant-duration pulses at the'plate of triode 62 which are integrated by resistor 63 and capacitor 64 to produce a varying voltage at the cathode of diode 65 related to the radiation intensity.

If this varying voltage falls below the voltage on the diode plate which is connected to divider 66, the diode conducts and a negative pulse is R-C coupled by capaci tor 67 and resistor 68 to the grid of normally conducting amplifier inverter 69. A positive pulse is thus produced at the plate of triode 69 whenever the radiation intensity exceeds a predetermined level, and is applied to a gating S.S.M. consisting of triodes 70 and 71.

. The negative going output signal produced at the plate of triode 71 whenever the S.S.M. is triggered over has a duration determined by the time constant of capacitor 72 and resistor 73. It is used as a delayed gating signal, and inhibits the action of the photocell/airblast circuit now to be described.

Light from lamp 74 normally falls on phototube 75. If the light is interrupted, the voltage developed across resistor 76 will be decreased. This negative going pulse is amplified'and inverted by triode 77 and applied to the control grid of pentode 78.

Pentode 78 is an and gate. A positive signal on the control grid will allow the tube to conduct and pass a negative pulse only if the gating signal on the suppressor grid is also positive. In this case the suppressor grid is tied to a voltage divider 79, the top end of which is connected to the plate of triode 71. Values are chosen to develop a gate closed signal only when triode 71 is conducting, i.e. when the gating S.S.M. is triggered over. If a deficiency of radiation pulses from the photomultiplier allows the normal gate open positive signal to remain on the suppressor of pentode 78, the tube conducts in response to the positive control grid signal, and a negative voltage is developed at the plate. This nega tive signal after inversion'in triode 80 is applied to the top of divider 81 which normally holds the grids of thyratrons 82 and 83 cut off. One of the thyratrons immediately fires. Power to operate the solenoid air blast valve is supplied through transformer 84 and controlled by these thyratrons arranged in a conventional full-wave rectifier arrangement.

The overall action of the circuit is thus to control the air blast in response to an interruption of the light beam, if such control is not inhibited by the overriding action of the radiation detector circuit. This simple electronic system demonstrates how the basic functional requirements may be fulfilled using conventional circuitry. The fundamental concept calls for a physical property monitor to produce a reject/ accept decision, and a separate timing control to actuate an air blast rejection mechanism.

It will be apparent that FIGURE 8 merely demonstrates the basic principles of an obvious electronic control system suitable to carry out the functions described, and that various refinements and elaborations thereof may be effected by those skilled in the art for the purpose of detecting a physical property by monitoring natural or induced quanta or radiation, such as gamma rays, neutrons, and primary or secondary X-rays.

Below the nozzle 17 is arranged a divider or splitter plate 26 located between the normal path or trajectory 13 and the trajectory 25. At the base of plate 26, a slide 27 receives the concentrated ore in the normal path and directs it onto a conveyor or the like 28 for conveyance to a point of further treatment, and a slide 27a receives the waste rock pieces and directs them onto a conveyor or the like 29 for discharging to waste.

While various types of quick-acting valves are suitable for use as the electrically actuated valve 20, a satisfactory form of such valve is illustrated in FIGURE 6. As shown, the valve includes a body 30 having an inlet 31 and discharge 32. The valve is of the diaphragm type and includes a diaphragm chamber 33 and a diaphragm 34 movable into engagement with a partition member 35 to close the valve and out of engagement therewith to open the valve. Chamber 33 is in controlled communication with air under pressure in pipe 18 through conduit 36 by means of a solenoid valve 37 having a solenoid 38 and a plunger 39 arranged to close or open port 40. Chamber 33 is also in controlled communication with the downstream side of valve body 30 by means of a solenoid valve 41 having a chamber 42, a port 43 leading from chamber 42 to diaphragm chamber 33, a port 44 leading from chamber 42 to the downstream side of body 30, and a solenoid 45 with plunger 46 arranged to simultaneously close and open ports 43 and 44.

Valve plunger 39 is normally biased in open position, solenoid 38 being in de-energized condition, to' open port 40 and place diaphragm chamber 33in communication with air under pressure through conduit 36. Valve plunger 46 is normally biased in closed position, solenoid 45 being in de-energized condition, to close ports 44 and 43. The air under pressure in the diaphragm chamber 33, because of the greater area of the diaphragm 34 as compared with the effective area of member 35, will thus actuate the diaphragm and close main valve 20. On energization of solenoids 38 and 45, plunger 39 will be moved to close port 40 and thus interrupt communication of the chamber 33 with air under pressure through conduit 36, and plunger 46 will be moved to open ports 44 and 43 and thus place the diaphragm chamber 33 in communication with the outlet end 32 of the valve body passage. Thus, the pressure in diaphragm chamber 33 will be immediately dissipated and the pressure of air in the valve body passage will move the diaphragm away from member 35 for substantially instantaneous opening of the valve body passage and passage of air under pressure therethrough from pipe 18 to nozzle 17.

It will be apparent that the method described may be applied to the treatment of any material which has a physical quality related to its value and which can be translated into electrical energy.

FIGURE 2 illustrates a modification of the invention suitable for use in the treatment of material such as rock wherein the ore and waste are of different colors. As shown, instead of the scintillation detector 14, a photoelectric cell 47 having a sensitivity peak at a particular portion of the color spectrum is employed. The remaining portion of the circuit may be substantially the same as shown in FIGURE 1.

FIGURE 3 illustrates a modification suitable for use in the treatment of material such as rock containing mineral which tluoresce when exposed to a certain Wave length of ultraviolet light. As shown, an ultraviolet light source 48 is provided together with a photoelectric cell 49 sensitive to fluorescent light. Again, the remaining stages of operation remain substantially the same.

A type of monitor readily available for detection of optical radiation is illustrated in the circuit of FIGURE 9;

The response of photomultiplier 85 follows a function of the incident light flux and develops an output across resistor 86. After amplification in stages 87 and 88, the signal output is fed to amplitude discriminator diode 65 which triggers a gating S.S.M. exactly as in the previously described radiometric system. A photoelectric cell may be used instead of the photomultiplier tube if desired.

Narrow-band light filters and appropriate choice of photo cathode will increase the difierential response to a particular wavelength and allow very sensitive discrimination of the difiuse light reflected from various colored articles.

If illuminated with ultra-violet radiations of a certain wavelength, some materials, for example scheelite, may be made to fluoresce, re-radiating energy as visible light of a certain color or wavelength which may be monitored by the circuit of FIG. 9. In this case visible light must be filtered from the light source and ambient light held to a minimum.

FIGURE 4 illustrates a modification which is useful in the treatment of materials such as rock containing minerals having magnetic properties. In this form of the invention, a coil of wire 5! with or without an induced magnetic field, constitutes the monitor. As shown, the coil is positioned whereby the rock stream falls through or near it. It will be apparent that a rock falling through coil 59 and having therein a mineral with magnetic properties will produce an electric current capable of controlling the subsequent photocell-air blast stage in desired manner.

A simple monitor circuit available for distinguishing ferromagnetic materials from non-ferromagnetic materials is shown in PEG. Stationary search coil 89 constitutes one arm of an A.C. balanced bridge circuit 90. A piece of ferromagnetic material passing through the energized loop disturbs the field in its vicinity and causes an unbalance current to flow through coil 91 across the arms. Inductive coupling transmits this signal to the grid of amplifier 92, where the negative excursions produce positive-going pulses at the plate. Pentode 93 amplifies and inverts these puises, and the negative-going output pulses are applied through capacitor 94 to trigger a S.S.M. composed of triodes 61 and 62 as in the radiometric system. Thereafter the integrating, gating and timing circuits may be as described previously and as shown in FIG. 8.

FIGURE 5 illustrates a modification wherein the varying penetration powers of materials towards X-rays depending upondensity and atomic weight is employed to effect the monitoring stage. As shown, 51 is a source of X-rays and 52 a photoelectric cell of either fluoroscope or radiation-sensitive type. The photocell-air blast stage may remain the same as in other modifications.

It will be apparent that the method and apparatus described results in great speed of treatment with high productivity. Moreover, the equipment involved is relatively inexpensive with low cost of operation and maintenance.- The various components of the equipment are of standard type. Inertia associated with purely mechanical devices is substantially eliminated in the present methodand apparatus. In conventional belt picking methods of sorting, the friction between the materials and the rubber of the belt is not comparable to the equivalent friction of the present invention involving the force of the air blast opposed by air.

It will be understood that the separation of trajectories in the present method is variable depending upon the force of air employed and the distance between the nozzle and the splitter plate. I

An important safety feature of the present invention resides in the fact that, iftor any reason the mechanism should fail, all of the material under treatment goes to the valve side of the splitter. It will also .be apparent that sizing and spacing of the pieces is not a critical condition in treatment in accordance with the present invention. Elimination of hand picking is an important feature in the handling of radioactive materials in order to avoid contamination.

In the monitoring stage, there is a substantially constant distance between the monitoring head and the pieces regardless of their size.

It will be appreciated that the timing of the air blast is precisely controlled by the photocell rather than by the monitoring device. The duration of the blast may be reduced to act only during part of passage of a piece of material if circumstances should render this step desirable.

Use of an air blast eliminates any necessity of touching the pieces by mechanism in the separation stage. Thus, there are no wearing parts except in the control valve. The force of the air blast is proportional to the size of the piece and is virtually unaffected by the shape of the piece itself.

We claim:

1. A method of sorting a mass of pieces of material to accept therefrom those of said pieces having a physical property related to value and to reject the remainder of said pieces which comprises providing a gravitationally falling stream of said mass of pieces arranged in a sub stantially single row, monitoring said physical property as said stream passes one point of its travel to detect said acceptable pieces, translating said monitored physical property into a first electrical signal, optically monitoring the transit time of each said piece in said stream past another point of its travel, deriving a second electrical signal directly related to said transit time from said optical monitoring step, directing an air blast having a time and duration responsive to said second electrical-signal across said stream at another point of its travel to change the trajectory of pieces impinged-thereby, and further controlling said air blast in response to said first electrical signal. 1 t

2. A method of sorting a mass of pieces of materialto accept therefrom those of said pieceshaving a physical property related to value and to reject the remainder of said pieces which comprises providing a gravitationally falling stream of said mass of pieces arranged in a substantially single row, monitoring said physical property as said stream passes one point of its travel to detect said d acceptable pieces, translating said monitored physical property into a first electrical signal, optically monitoring the transit time of each said piece in said stream past another point of its travel, deriving a second electrical signal directly related to said transittime from said opti cal monitoring step, directing an air blast having a time 'and duration responsive to said second electrical signal across said stream at another point of its travel to change the trajectory of pieces impinged thereby, and inhibiting said air blast in response to said first electrical signal to permit continuance of certain of said pieces in unchanged trajectory.

3. A method of sorting a mass of pieces of material to accept therefrom those of said pieces having a physical property related to value and to reject the remainder of said pieces which comprises providing a gravitationally falling stream of said mass of pieces arranged in a substantially single row, monitoring said physical property as said stream passes one point of its travel to detect said acceptable pieces, translating said monitored physical property into a first electrical signal, optically monitoring the transit time of each said piece in said stream past another point of its travel, deriving a second electrical signal directly related to said transit time from said optical monitoring step, directing an air blast having a time and durationresponsive to said second electrical signal across said stream at another point of its travel to change the trajectory of pieces impinged thereby, and inhibiting said air blast in response to said first electrical signal to permit continuance of said acceptable pieces in unchanged trajectory and changing the trajectory of said rejectable pieces only by said air blast.

4. A method of sorting a mass of rock pieces some of which contain valuable amounts of radioactive mineral to accept therefrom those of said pieces containing said valuable amounts of radioactive mineral and to reject the remainder of said pieces which comprises providing a gravitationally falling stream of said mass of pieces arranged in a substantially single row, receiving radioactive mineral-identifying radiations from rock pieces as said stream passes one point of travel to detect said acceptable pieces, translating said radiations above a predetermined intensity into a first electrical signal, optically monitoring the transit time of each said piece in said stream past a lower point of its travel, deriving a second electrical signal directly related to said transit time from said optical monitoringstep, directing an air blast having a time and duration responsive to said second electrical signal across said stream at a still lower point of its travel to change the trajectory of pieces impinged thereby, and inhibiting said air blast in response to said first electrical signal to permit continuance of said acceptable pieces in unchanged trajectory and changing of the trajectory of said rejectable pieces only by said air blast.

5. Apparatus for sorting materials which comprises a conveyor for pieces of material to be sorted having a discharge end for discharging said pieces in a gravitationally falling stream, the space below said discharge end being substantially unobstructed to provide a substantially vertical path forfree fall of said stream, a monitoring device arranged opposite said path for generation of a first electrical signal in response to a physical property related to value in certain of said pieces as they move past said monitoring device, a photoelectrical device arranged opposite said path below said monitoring device for generating a second electrical signal related to the transit time of each said piece in said stream as it moves past said photoelectric device, an air blast nozzle arranged'opposite said path below said photoelectric device and directed across said path, means for supplying air under pressure to saidnozzle, a valve controlling flow of air from said air supplying means to said nozzle, an electrical connection between said valve and said photoelectric device to open said valve in response to said second electrical signal and cause a blast of air from said nozzle for changing the trajectory of pieces impinged thereby, and an electrical connection between said valve and said monitoring device to close said valve in response to said first electrical signal and interrupt said blast of air.

6. Apparatus for sorting materials which comprises, a conveyor for pieces of material to be sorted having a discharge end for discharging said pieces in a gravitationally falling stream, the space below said discharge end being substantially unobstructed to provide a substantially vertical path for free fall of said stream, a monitoring device arranged opposite said path for generation of a first electrical signal in response to a physical property related to value in certain of said pieces as they move past said monitoring device, a photoelectric device arranged opposite said path below said monitoring device for generating a second electrical signal related to the transit time of each said piece in said stream as it moves past said photoelectric device, an air blast nozzle arranged opposite said path below said photoelectric device and directed across said path, means for supplying air under pressure to said nozzle, a valve controlling flow of air from'said air supplying means to said nozzle, an electrical circuit connecting said valve and said photoelectric device to open said valve in response to said second electrical signal and cause a blast of air from said nozzle for changing the trajectory of pieces impinged thereby, and an electrical circuit including said first electrical circuit connecting said monitoring device and said valve to close said valve in response to said first electrical signal and interrupt said blast of air.

7. Apparatus for sorting materials which comprises, a conveyor for pieces of material to be sorted having a discharge end for discharging said pieces in a gravitationally falling stream, the space below said discharge end being substantially unobstructed to provide a substantially vertical path for free fall of said stream, a device responsive to radiations of predetermined intensity arranged opposite said path to receive value identifying radiations from certain of said pieces as they move past said radiation responsive device, an optical monitoring device having a source of illumination and a photoelectric detector arranged on opposite sides of said path below said radiation responsive device, an air blast nozzle arranged opposite said path below said detector and directed across said path, means for supplying air under pressure to 'said nozzle, a valve controlling flow of air from said air supplying means to said nozzle an electrical connection between said detector and said valve to open said valve in accordance with the transit time of each said piece past said detector and cause a blast of air from said nozzle for changing the trajectory of pieces impinged thereby, and an electrical connection between said valve and said radiation responsive device to close said valve in response to reception of value identifying radiations in said radiation responsive device and interrupt said blast of air.

8. Apparatus for sorting materials which comprises, a conveyor for pieces of material to be sorted having a discharge end for discharging said pieces in a gravitationally falling stream, the space below said discharge end being substantially unobstructed to provide a substantially vertical path for free fall of said stream, a device responsive to radiations of predetermined intensity arranged opposite said path to receive value identifying radiations from certain of said pieces as they move past said radiation responsive device, an optical monitoring device having a source of illumination and a photoelectric detector arranged on opposite sides of said path below said radiation responsive device, an air blast nozzle arranged opposite said path below said detectorrand directed across said path, means for supplying air under pressure to said nozzle, a valve controlling flow of air from said air supplying means to said nozzle, electrical means for opening and closing said valve, an electrical connection between said detector and 9 said electrical means to open said valve in accordance with the transit time of each said piece past said detector and cause a blast of air from said nozzle for changing the trajectory of pieces impinged thereby, and an electrical connection between said radiation responsive device and said electrical means to close said valve in response to reception of value identifying radiations in said radiation responsive device, and interrupt said blast of air.

9. Apparatus for concentrating radioactive ore which comprises, a conveyor for rock pieces to be sorted having a discharge end for discharging said pieces in a gravitationally falling stream, the space below said discharge end being substantially unobstructed to provide a substantially vertical path for free fall of said stream, a scintillation counter responsive to radioactive radiations of predetermined intensity arranged opposite said path for generation of a first electrical signal in response to said radiations in certain of said pieces as they move past said scintillation counter, a photoelectric device arranged opposite said path below said scintillation counter for generating a second electrical signal related to the transit time of each said piece in said stream as it moves past said photoelectric device, an air blast nozzle arranged opposite said path below said photoelectric device and directed across said path, means for supplying air under pressure to said nozzle, a valve controlling flow of air from said air supplying means to said nozzzle, electrical means to open and close said valve, an electrical connection between said electrical means and said photoelectric device to open said valve in response to said second electrical signal and cause a blast of air from said nozzle for changing the trajectory of pieces impinged thereby, and an electrical connection between said electrical means and said scintillation counter to close said valve in response to said first electrical signal, and interrupt said blast of air.

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